1. Field of the Invention
The present invention relates to a dual-band voltage-controlled oscillator, and more particularly to a dual-band voltage-controlled oscillator having less interference between oscillators and improved phase-noise characteristics by using bias switching and output-buffer multiplexing.
2. Description of the Related Art
As wireless communication services advance, next-generation wireless communication services such as wireless local loop (WLL) and International Mobile Telecommunications 2000 (IMT-2000) will be launched in the future together with the digital mobile phones and personal communication services (PCSs) in commercial services. The next-generation wireless communication services require diverse functions such as high-capacity data processing in image transmission systems.
The wireless communication services use different frequency bands. For example, a digital mobile phone uses the 800 MHz frequency band, a PCS uses the 1.9 GHz frequency band, IMT-2000 uses the frequency band of 1.8˜2.1 GHz, and next-generation wireless communication services use the frequency band of 2.2 GHz.
The RF transmission/reception unit of the next-generation wireless communication system has a voltage-controlled oscillator providing radio frequencies (RF). The voltage-controlled oscillator refers to an oscillator having oscillation frequencies linearly changing according to input control voltages.
A conventional dual-band voltage-controlled oscillator will be described. FIG. 1A and FIG. 1B are views for showing a structure of a conventional dual-band voltage-controlled oscillator. First, FIG. 1A is a view for showing a structure of a dual-band voltage-controlled oscillator in which the common source terminal of a cross-coupled differential amplifier has a frequency two times as high as an output frequency. Such a dual-band voltage-controlled oscillator has a problem in that an output signal at the common source terminal (node A) has a small magnitude and poor phase noise characteristics due to interference between frequencies.
The dual-band voltage-controlled oscillator shown in FIG. 1B generates a frequency of 6 GHz and a frequency of 3 GHz obtained by dividing the frequency of 6 GHz in half by using the 6 GHz voltage-controlled oscillator. The above structure has a problem that its overall area is increased since additional circuits such as mixers and notch filters are required.
In addition, there is a method of obtaining a dual-band frequency by switching an array of inductors and capacitors of an LC tank. Such a method has difficulty in implementing resonant characteristics of the LC tank since values of the inductors and capacitors need to change in a wide range and MOS switches are added in order to obtain a frequency two times higher than an oscillation frequency. Further, the method has a problem of increased chip area due to the addition of inductors and capacitors.